Nitrogen addition experiments were conducted in three plant communities in a post-mined peatland and on the surrounding original Sphagnum bog in northern Japan to compare the effects of nitrogenous fertilization among the plant communities. Gradients of added nitrogen were prepared by adding nitrogen to 1 m x 1 m plots at rates of 0-36 g/m(2)/yr. These different levels of nitrogen addition were applied to four sites representing a vegetation development chronosequence: bare ground (BG), a Rhynchospora alba sedgeland (RA), a Moliniopsis japonica grassland (MJ) and the original Sphagnum bog (SS). Vegetation was monitored in each plot for three years, and the groundwater level and nitrogen concentration in peat-pore water were monitored in each plot. The nitrogen concentration in peat-pore water increased with increasing nitrogen addition on BG, but was constantly low at the vegetated sites, suggesting the occurrence of nutrient uptake by plants. Species richness decreased as a result of nitrogen addition at the SS site, where plant cover was high. The aboveground biomass of the two common grasses M. japonica and Phragmites communis was not influenced by nitrogen addition in the post-mined peatland. Grasses and sedges, which produce large underground organs, showed increased biomass as a result of nitrogen addition in the SS plot whereas Sphagnum declined. Additionally, forbs declined in the SS plot with increasing nitrogen, most likely due to competition with the increased populations of grasses. Therefore, nitrogen enrichment can promote the dominance grasses, leading to a decrease in mosses and forbs.